Recently, a technique to increase transmission capacity in optical transmission is being actively studied. In particular, regarding optical transmission systems (metropolitan type systems) in urban areas, the number of fibers in an optical fiber cable tends to increase more and more. Recently, an optical fiber cable of approximately 1000 fibers has been put into practical use.
To improve workability in splicing and/or connecting, optical fiber ribbons are generally adopted for such an optical fiber cable. In an optical fiber ribbon, a plurality of coated optical fibers are arranged in a parallel planar array and integrated by a resin. At present, optical fiber ribbons of 4, 8, 12, 24 fibers, and the like are in practical use.
Further, coated optical fibers used for an optical fiber ribbon can be obtained by coating an optical fiber having a diameter of approximately 125 μm with an ultraviolet curing resin to have a diameter of approximately 250 μm. The optical fiber is usually a single mode optical fiber (SMF) having zero dispersion at a wavelength around 1300 nm.
On the other hand, as a technique to increase transmission capacity in optical transmission, wavelength division multiplexing optical transmission system (WDM system) is actively studied, particularly for long distance transmission systems. Employment of the WDM system also comes to be considered for metropolitan type systems of relatively short distances.
Generally, to perform WDM optical transmission, it is necessary that a waveform distortion that can hardly be reshaped does not occur. For that purpose, it is deemed effective to suppress nonlinear phenomenon in an optical transmission line. While the optical transmission line locally requires appropriate dispersion, it is necessary to minimize dispersion for the optical transmission line as a whole. Further, any dispersion difference between the wavelengths for optical signals causes a difference in waveform distortion amount for each wavelength, so that it is necessary to make the dispersion slope in the optical transmission line small.
To satisfy these requirements, when performing WDM optical transmission, there is adopted in many cases an optical transmission system wherein the dispersion is controlled by employing an optical fiber with positive dispersion (hereinafter referred to as a positive dispersion optical fiber) and an optical fiber with negative dispersion (hereinafter referred to as a negative dispersion optical fiber) in combination. In particular, such an optical transmission system is widely adopted for long distance transmission systems.
In a metropolitan type WDM system for a relatively short distance, the required characteristics are not so strict compared to the case for a long distance transmission system. For example, in a metropolitan type WDM system, the maximum transmission speed is approximately 10 Gbps at most, and the maximum transmission distance is approximately 40 km, so that an optical amplifier is not required different from the case for a long haul transmission system.
Generally, in dense wavelength division multiplexing (DWDM) optical transmission of a transmission speed of 10 Gbps, the permissible maximum cumulative dispersion (dispersion proof-stress) of the optical transmission system is approximately 500 ps/nm, which is relatively large. Thus, what is required is an inexpensive and simple optical transmission system, rather than a system with strictly controlled dispersion characteristics.
When an optical transmission line using an SMF is adopted in a metropolitan type WDM optical transmission system, the dispersion value of an SMF is approximately 17 ps/nm/km at a wavelength of 1550 nm. Thus, in the case of a transmission distance of 40 km, the cumulative dispersion of the optical transmission line becomes approximately 680 ps/nm, which exceeds the dispersion proof-stress (500 ps/nm) of the system, resulting in a deterioration of a transmission quality. Thus, it becomes necessary to control the dispersion characteristics by using a negative dispersion optical fiber, such that the cumulative dispersion of the optical transmission system does not exceed the dispersion proof-stress. A dispersion compensating optical fiber (DCF) is generally used as a negative dispersion optical fiber for controlling the dispersion characteristics of an optical transmission line formed by SMF.
However, in a conventional DCF module, each DCF is accommodated in a module, and connectors are attached to both ends of the DCF, to allow each DCF being connected to an optical fiber such as SMF of an optical transmission line. Thus, connecting a module for each DCF of an optical fiber cable with numerous fibers (with e.g., 1000 fibers) results in the accommodation space becoming rather bulky. Further, the connecting operation takes much time with the result that the cost of the optical transmission system becomes high.